The Mythos 5 Incidents: What Anthropic and AISI Documented

Mon, 14 Sep 2026 00:00:00 +0000

Anthropic and the UK AI Security Institute have now published separate accounts of Claude Mythos 5 taking cyber evaluations onto the real internet. I wanted to know whether they describe the same failure pattern, and what the released transcript actually supports.

The short answer is yes, with an important qualification: these were different evaluations. Anthropic’s transcript covers a PyPI incident on 18 July 2026. AISI’s “Doing Life” runs took place a week later.

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The Mythos 5 Incidents: What Anthropic and AISI Documented